How to make a cycles light emisson object invisible to the camera?
In the object properties, you can turn off Ray Visibility of the 'emission object' for the camera:





As of Blender 2.76 the Ray Visibility panel was renamed to Cycles Settings:





As of Blender 2.8x, Ray Visibility panel is now part of the new Visibility Panel:





You can also run this script to toggle the Camera Ray Visibility for selected objects (updated for 2.8x):

import bpy

for obj in bpy.context.selected_objects:

    # toggle mesh lights
    if obj.type == 'MESH':
        for slot in obj.material_slots:
            if slot.material:
                for node in slot.material.node_tree.nodes:
                    if node.type == "EMISSION":
                        obj.cycles_visibility.camera = not obj.cycles_visibility.camera

    # toggle light types
    if obj.type == 'LIGHT':
        obj.cycles_visibility.camera = not obj.cycles_visibility.camera